Alloys and processes for their manufacture

ABSTRACT

BULK ALLOYING PROCESS FOR THE PRODUCTION OF A MULTIPHASE ALLOY IN THE FORM OF AN ENGINEERING MATERIAL COMPRISING THE STEPS OF INTRODUCING AT LEAST ONE OF THE CONSTITUENTS INTO A CONTROLLABLE VACUUM OR LOW PRESSURE SYSTEM IN SOLID FORM, EVAPORATING THE SAID CONSTITUENT OR CONSTITUENTS IN A CONTROLLABLY HEATED SOURCE MEANS AND DEPOSITING THE VAPOUR ON A COLLECTOR MEANS AND ALLOYS PRODUCED THEREFROM.

R. l.. Bickn-ithilks P-'FL 3,554,739

Jan. 12, 1971 .LLOYS AND PRooEssEs'FoR THElR MANUFACTURE Filed sept, e. 41967 4 Sheie'cs-Sheet 2 52.5200 028mm Qwumzw Of. o

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United States PatentI Office 3,554,739 Patented Jan. 12, 1971 3,554,739 ALLOYS AND PROCESSES FOR THEIR MANUFACTURE Robert Lewis Bickerdike, Francis Julian Bradshaw, Garyth Hughes, and William Norman Mair, Farnham, and Harry Christopher Ranson, Farnborough, England, assignors to Minister of Technology in Her Britannie Majestys Government of the United Kingdom of Great Britain and Northern Ireland, London, England Filed Sept. 6, 1967, Ser. No. 665,844 Claims priority, applicationGreat Britain, Sept. 7, 1966,

39,928/ 66 Int, Cl. CZZc l/ U.S. 'Cl. 75-135 5 Claims ABSTRACT OF THE DISCLOSURE Bulk alloying process for the production of` a multiphase alloy in the form of an engineering material comprising the steps of introducing at least one of the constituents into a controllable vacuum or low pressure system in solid form, evaporating the said constituent or constituents in a controllably heated source means and depositing the vapour on a collector means and alloys produced therefrom.

'This invention relates to alloys and processes for their manufacture.

A common method of obtaining high strengths in metals is by dispersion hardening. A line enough dispersion of a suitable strong phase in the metal matrix raises the yield stress.

The dispersion is usually obtained by precipitation from a supersaturated solid solution, according to the well known precipitation hardening process. It can also be obtained by mechanical and/ or chemical methods followed by powder metallurgy, or by diffusion (internal oxidaton). These methods in general have limitations which have restricted the development of useful engineering alloys. For example chemical and diffusion methods are restricted to particular systems and mechanical methods tend to give irregular dispersions. The precipitationfrom-solid-solution method is based on the phase diagram of the alloy system; only those elements which can dissolve in the matrix metal to a sufficient extent at the solution-forming temperature, and which have a solubility limit decreasing suiciently sharply with decreasing temperature, can be used (with or without the solvent metal) to form a useful amount of precipitated material. An element which can be used to form precipitated material may be termed a hardening constituent. The amounts and types of precipitates formed by this technique are therefore limited. For example aluminum in the solid state is a relatively poor solvent for other metals and thus precipitation hardened aluminum alloys are relatively difficult to obtain.

The object of the present invention is to provide high strength alloys and processes for their manufacturein which the solubility restrictions referred to above are largely, if not wholly, avoided.

According to the present invention a bulk alloyed process for the production of a multi-phase alloy in the form of an engineering material comprises the deposition from vapour of the constituents on a collector within a low pressure or Vacuum system. Normally at least one of the constituents is evaporated from a source meanswithin the said vacuum system.

An engineering material produced in accordance with the present invention is a deposit having a thickness greater than 0.01 inch and capable of being removed from the collector and being worked into sheet, strip or other with the present invention conveniently comprises a vessel containing a controllably heated evaporative source means, a temperature controllable collector and means for controlling the pressure within the vessel. The apparatus may also include a shutter between the collector and the source or sources and include measuring means for monitoring the background gas pressure in the vessel, heat input to and the temperature of the source means, evaporation and deposition rates.

All of the constituents of the eventually formed multiphase alloy may be evaporated or one or more constituents may be gaseous and trapped or absorbed and then trapped by the deposited material; for example oxygen or a volatile compound such as a metal carbonyl.

When more than one constituent is to be evaporated each constituent may be evaporated from one or more separate sources or a mixture of constituents may be evaporated from the same source or sources. Multi-phase material may be produced by successive deposition of the constituents, or by simultaneous deposition from a vapour stream with fluctuating composition across the collector surface.

Where two or more metal constituents are simultaneously evaporated from a single source small individual volumes of an alloy of suitable composition may be fed to the evaporation position within the evacuated vessel to comprise the evaporative source so that each individual increment of deposit has the required composition. In another method of achieving simultaneous deposition, the evaporative source may comprise a larger volume of alloy which is heated to achieve a steady state at which the melt surface of the source assumes a composition such that the constituents evaporate from it in the desired proportions. The composition of the melt surface and the evaporating stream will in this case be different. The steady state may be achieved by feeding into the melt a 'feed stock of the same composition as the vapour stream.

Methods of heating the source means include radiation from a high frequency susceptor, eddy current heating of the charge itself either with or without levitation, radiation from a resistance heater (conveniently made from one of the refractory metals or other electrically conducting material), direct resistance heating of either the charge itself or its container, electron beam heating, plasma beam heating or arc heating. Methods which put heat directly into the charge, for example electron beam heating, have the advantage of reducing the severity of problems arising from chemical interaction between the charge and its container.

The choice of heating method is determined by the material that is to be evaporated, the desired rate of evaporation, chemical interactions, the temperature and surface area of the charge and the thermal efficiency. For example, aluminum can be evaporated from a radiation heated vitreous carbon boat or crucible as described in detail below. Higher aluminum evaporation rates can be obtained either by using an electrically heated bar of conducting refractory material (for example boron nitride or titanium diboride) or by electron beam heating of a charge supported by a thin layer of thermally insulating refractory material on a water cooled copper hearth. Metals of high vapour pressure, for example, manganese may be evaporated or sublimed from the solid state.

It is desirable to collect the deposit on a surface with which it is in good thermal contact, as the substrate temperature can have a pronounced effect on the structure of the deposit. It is also necessary to have a fairly smooth substrate, because surface roughness results in growth features which can give rise to the deposit. One aspect of this is the elimination of dust or grit particles from the collector surfaces after it has been polished and cleaned. Another is the avoidance of particles of metal blown from the evaporation source onto the collector during melting or degassing.

Deposits may be under a stress, usually tensile in nature, and it is necessary to ensure that this does not result in the deposit being prematurally pulled away from the collector.

This can be avoided if the adhesion of the deposit is great enough. However it is necessary to separate the newly deposited material from the collector, and to do this the following methods may be used.

The built-in stress in a thick deposit tapered to zero thickness gradually at the edge is usually insufficient to pull it away from the collector, but a sharp change in thickness introduced by scratching or cutting the deposit causes a crack to run between the deposit and the collector under suitable conditions. It is more dicult however to start such a crack without damaging the collector if the deposit is hard, but a multiple collector, i.e. a collector to which there is a closely tting annulus may be used. The deposit forms on the central region and on the annulus, and when a thick enough deposit has been obtained, the annulus can be moved forward relative to the rest of the collector pushing the deposit off the central region. Separation is assisted in both these cases if the central region of the collector is covered with a thin lm of oxide or carbon or some other material which will limit adhesion, leaving a clean annular region free for good adhesion.

Another technique is to solder a thin metal sheet to the collector surface with a solder of suita-ble melting point. Deposition takes place on the sheet. When it is complete, the solder is melted and the deposit and sheet are removed. After the remaining traces of solder have been removed, the sheet can if desired be dissolved in the deposit during subsequent heat treatment to provide additional alloying. For example this can be used in the aluminum-copper system.

It is also proposed to make the collector of the same material as the deposit; using suitable conditions to obtain good adhesion. After deposition the combined collector-cum-deposit may be hot worked to the original collector thickness and the surplus material cut off.

The collector material is chosen in part for its heat transfer properties and for its thermal expansion. When depositing aluminum-copper collectors have been used in the temperature range 196 to 220 C., and aluminum collectors in the range 65 to 350 C. Deposited aluminum has also been collected on stainless steel up to 550 C. and on molybdenum. A wide range of metals and alloys can be considered for collectors, adhesion to, or interaction with, the deposit being a relevant aspect. It may also be feasible and convenient to collect the deposit directly on a fusible or soluble layer on a metal surface, for example a thin deposit of an inorganic salt or compound.

The collector may be stationary but in order to even out irregularities caused by non uniform evaporation from the source or sources the collector and the source or sources may be moved with respect to one another during the deposition. Such motion may be translational or vibrational but in the preferred embodiment the collector comprises a cylinder which is rapidly rotated over an array of sources; for example the evaporating materials may be contained in heated troughs lying parallel to the axis of rotation of the drum and close to the drum surface. A friable deposit suitable for powdering, containing multilayers of two or more phases may be obtained in this way. The thickness and separation of the phases is determined by the rates of deposition and the speed of rotation of the dlum. Another possible variety of moving collector is a rotating plate collector, with evaporating troughs below it.

By this means a deposit may also be obtained in which the compositional fluctuations are very slight and the multi-phase alloy produced has substantially the same composition throughout. This technique is particularly useful when the source is a melt of metals having markedly different vapour pressures. For example, the hot area on the surface of a melt of aluminum and iron or titanium being heated by an electron beam will not all be at the same temperature and the ratio of the two metals evaporating will vary across the hot area. Also high evaporation rates allow very little mixing in the vapour beam and a rotating cylindrical collector is the preferred method of obtaining uniform composition of the deposit.

Processes in accordance with this invention provides thick plate-like or slab deposits of 0.01 inch or more in thickness. These deposits may be removed from the collector and worked into sheet, strip or other wrought form and heat treated before, during or after working to produce desired mechanical properties. Alternatively the deposits may be produced for processing by powder metallurgy, either by powdering the deposit or arranging the deposition conditions, for example, the angle of incidence, so that the deposit is easily broken up.

When making massive adherent deposits alternate or successive deposition can be employed, either by altering the composition of the vapour stream, or interrupting it, or changing the pressure or composition of the permanent or residual gases or vapours in the system, or the substrate temperature, or some other deposition condition, so as to obtain layers with differing compositions or structures, or for some other purpose. This can be combined with the formation of the main layer by codeposition. For example in some cases it has been found desirable Iwhen depositing aluminum alloys to start depositing with the substrate above 300 C. in order to obtain adequate adhesion to the collector, before depositing the main layer at a lower temperature in order to obtain the desired alloy structure. Other preliminary deposits can be used to regulate adhesion. Some deposits are porous, and it can be desirable to put down an impermeable layer to seal this porosity before exposing the deposit to the atmosphere. Alternatively heating in vacuum at the sintering temperature may also reduce the porosity. A further example is where it is found convenient to incorporate an alloying constituent not by co-deposition but by deposition before or after or at some stage in the formation of the main deposit, followed by a diffusion heat treatment.

Processes in accordance with the present invention are particularly useful for permitting the production of dispersion hardened alloys in which the strong dispersed phase constitutes a greater proportion of the final alloy than would be predicted on the basis of the phase diagram of the alloy system when the alloy is produced by the regular precipitation-from-solid-solution method.

In accordance with one embodiment of the invention a process for the production of a multi-phase alloy in the form of an engineering material comprises the deposition from vapour of the constituents on a collector within a vacuum or low pressure system, whereby the product is a dispersion hardened alloy in which the proportion of hardening constituent in the dispersion hardened alloy is greater than the proportion of hardening constituent in a solid solution with the matrix metal.

Normally at least one of the constituents is evaporated from a heated source means Within the said vacuum or low pressure system.

By way of example suitable apparatus for carrying out processes in accordance with, the present invention will now be described together with application of these processes to the production of dispersion hardened aluminum alloys.

`Aluminunihas a reasonably large equilibrium solid solubility for copper, silver, magnesium and zinc only of the commonly available alloyingv elements and of these copper is only soluble to the extent of 5.65% at the enutectic temperature. Other common alloying additions such as iron, nickel, manganese are only soluble to 0.05, 0.05` and 1.82% respectively, and chromium to 0.77% in their binary systems 'with aluminum. Oxygen is not normally found Vin solid 'solution in aluminum and has taken no direct part in precipitation hardening according to known practice. In accordance with the present invention and 'as described Ibelow it is possible to operate outside these solid solubility limitations by deposition from vapour methods so that a useful age hardening effect can be obtained from precipitated phases containing elements such asV oxygen, copper, iron and chromium and so that elements'which are already included in precipitates, e.g. copper, silicon and manganese, 'can conveniently 'be included in mu'cli` larger amounts than the equilibrium diagrams suggest. It is also possible to make alloys containing phases, such as' alumina, which are formed by nucleation nd growth Aduring deposition, as an alternative to or in additiontoprecipitation from solid solution.

' The element in the precipitated phasev that' impart the hardening may be termed the hardening constituent, and more than one hardening 'constituent may be included in the precipitated phase. l

In the'accoinpanying drawings FIG. 1 is a schematic illustration" o'f` an apparatus suitable for carrying out processes in accordance-with the invention.

FIG; 2 illustrates a cross-section of a particular type of evaporative source, v `FIG. 3 is a graph" of the lattice parameter for aluminium plotted against 'proportions "'of iron, chromium, copper and oxygen that ,may be obtained in dispersion hardened aluminium alloys, Y "FIG, 4 shows'the 'change of-microhardness with time on -annealin'gan aluminum-oxygen alloy at 400 C., and FIG. :'5Vsho'w`s' the-"change of 'microhardness with time 'on annealing an aluminum-chromium alloy at 350 C.

In FIG.' 1 the sourceineans l'comprises a carbon boat supported on a thin graphite plate 11 above a tungsten rod hleateiml'lZ, this assembly being surrounded by a set oimolybde'numv radiatjion screens 13'.` A moveable shutter 14 isprovided between the source 1 and4 a collector 15. The ycollector 15 forms part of the vacuum vessel 1 6, which is'evacuatedfbyv meansof a.. standard oil diffusion pump system ,17, while the pressure is monitored by an ionisationpress'ure gauge 18. Electrical power is supplied to the tungsten rod heater. 12L by electrical leads 19 through a seal 2 0 inthe vacuum envelope. A spoon 21 operated through a seal A22. is used to place the charge in vthe boat 10. A handle 23 'operatingthrough a seal 24 is provided to `remove-"the shutter 14 from its position between the source'l and the "collector 15. The collector 15 mayI be hollow and 'c'ooled eithergby circulating cold water through it or by UI lling it` with a suitable cooling mixture for exampleacetonelsolid carbon dioxide. Alternatively 'the collector' may'fbe heated to a required temperature by any Suitblemeans; for vexample by playing a blow-pipe on theouter surface. A needle valve 25v is providedV through which gases may be added to the system. W y

In operation the tungsten heater 12 is raised to a high temperature in vacuo-,.with the lshutter 14 in position. After a short time the'power is reduced and when the pressure has reached about 10*rl torr the boat 10 is heated to about the melting point of the material to be evapo- 6 rated. Metal pellets are introduced, and after melting has occurred the boat 10 is heated to the required evaporation temperature. When the residual gas pressure has fallen to a satisfactory level, or when the required gas pressure has been obtained when a gas is being deposited, the shutter 14 is opened and the deposit collected on the collector 15. The shutter 14 is closed when the required amount has been evaporated.

In the alternative source means illustrated in FIG. 2 a vitreous carbon crucible 30 is supported upon three hollow molybdenum legs 3-1, one of which contains a tungsten-iridium thermocouple 32 to give an indication of the source temperature. Heat is supplied by a tantalum sheet heater 33 inside molybdenum radiation screens 34. As a further modification two crucibles may be placed one inside the other to provide a source for the simultaneous evaporation of two separate constituents as is illustrated by the chain-dotted line 35 in FIG. l and FIG. 2.

The lower portion of FIG. 3 shows three straight line graphs showing the variation of aluminum lattice paramater with the concentration of solute copper 40. chromium 41 and iron 42. The solid portion of each of them represents the solubility of the second phase. It should be noted that iron has such a low solubility; 0.05% that a sol-id portion cannot readily be shown for it. The points 43, 44 and 45 represent respectively concentrations of copper, chromium and iron in aluminum that can be obtained by processes in accordance with the present invention.

The upper half of the graph shows the variation of aluminum lattice parameter with oxygen concentration in alloys produced in accordance with the present invention.

The following examples are illustrative of the present invention:

EXAMPLE 1.-ALUMINUM-CHROMIUM A double crucible source 35 was used with 112 g. of 99.99% pure aluminum in the inner crucible and l5 g. of pure electrolytic chromium flake in the annular space; the collector was of polished copper. The apparatus was evacuated and the collector 15 heated to 210 C. The source was heated to the evaporation temperature of 1550" C., the shutter 14 opened and the collector 15 cooled rapidly to 20 C. Deposition was continued for about 40 minutes during which time the pressure was about 10-5 mm. Hg and a deposit 0.012 thick collected. The chromium content and lattice parameter of this deposit is shown as 44 on FIG. 3 and the change of microhardness on annealing at 350 C. in argon is shown in FIG. 5.

EXAM-PLE 2.-ALUMINIUMIRON An iron aluminum alloy containing 62% aluminum and 38% iron was heated in vacuo in a vitreous carbon boat 10 by a tungsten rod heater 12 as shown in FIG. l. When the metal had melted the shutter 14 was opened and the source heated to the evaporation temperature of 1360 C. A deposit weighing 0.12 g. was obtained on the collector at 20 C. The iron content and lattice parameter are shown as 45 in FIG. 3.

EXAMPLE 3 .-ALUMINIUM-COPPER Thiswas carried out as for Example 2 except that an aluminium-copper alloy containing about 50% copper was used and the evaporation temperature was 1400 C. The composition of the deposit and its lattice parameter are shown as 43 in FIG. 3.

7 EXAMPLE 4.--ALUMINIUM-OXYGEN dispersions lose microhardness on annealing, see Table DEPOSITS 2 below, items 4 to 7 inclusive. Oxygen was obtained as an alloying constituent in 75--138; 117-107 TABLE 2 Oxide Oxygen content, Initial Alum. evap. pressure, Collector wt. hardness, temp., C. X10-i torr temp C. percent kg. mm.-z After annealing 17 -65 14. 5 130 After 40 hrs. at 440 C.,150 kg. mmf?. 20 20 13 102 10 maar 450 C.,140kg.mm.2. 10 100 3 154 40 maar 440 C.,156kg.mm.2. 177-210 7 182 After 40 hrs. at 440 C., 100 kg. mmf?. After 40 hrs. at 440 C., 151 kg. mmre. 17 100 1-1 232 After 3 hrs. at 450 C., 120 kg. mnt-2.

aluminum by evaporating aluminum broadly as described 15 What is claimed is: using the single source illustrated in FIG. 2 in a low 1. A bulk alloying process for the production of pressure system in which the residual gas was oxygen. a multiphase alloy in the form of a sheet of engineering The pressure of oxygen in the system is determined by material comprising the steps of introducing at least the rate of evaporation required and the composition of one of the constituents into a controllable vacuum or alloy being produced. Generally the pressure of oxygen 20 low pressure system in solid form, evaporating the said will not exceed one fth of the vapour pressure of the constituent or constituents in a controllably heated source aluminium at the evaporation temperature. The table bemeans, introducing at least one other constituent into the low gives details of the evaporating temperature recorded said vacuum or low pressure system in the form of a being that of the thermocouple in the leg of the crucible gas, depositing the vapour on a temperature controllable support. The composition of the alloy produced was decollector means, the deposition being carried on for a termined by chemical analysis and also `by X-ray intensity length of time such that the alloy deposited attains a measurement in certain instances. A plot of composition thickness of at least 0.01 inch and removing the alloy against lattice parameter is given on the top half of from the collector, in aform capable of sustaining normal FIG. 3. metallurgical working.

2. A bulk alloying process in accordance with claim 1 in which the principal constituent of the alloy is aluminum and a further constituent is oxygen. TABLEl 3. A bulk alloying process in accordance with claim Aluminum Conector 1 in which aluminum is introduced into the controllable evaporatim, Oxygen temper, Refcmme vacuum or low pressure system in solid form as the matelperatm-e, L )esture atprce. nurxeralor trix metal of the alloy and oxygen is also introduced X o gum therein, the pressure of oxygen in the vacuum or low pressure system being not greater than one fifth of the 1.0 20 46 3 0 20 47 vapour pressure of the aluminum at the temperature of leg gg g 40 the controllably heated source means. 15:0 20 50 4. A bulk alloying process in accordance with claim im :22 3 in which the temperature of the temperature controllable 10:0 100 53 collector means is at or below ambient temperature and the temperature of the temperature controllable source means is between about 1200 and 1600 C., and the deposit produced is annealed at a temperature above about 350 C.

In FIG. 3 the symbol G) indicates a chemical deter- 5. A bulk alloying process in accordance with claim mination and X indicates an X-ray intensity determina- 3 in which the temperature of the temperature controltion. lable collector means is lbetween about 100 and 500 C.,

the temperature of the temperature controllable source EXAMPLE 5,-AGE-HARDENED DEPOSITS means is between 1200 and 1600 C., the deposit produced is annealed at a temperature -between about 500 and 630 Suitable heat treatment 0f aitlffllmim-Oxygeii deposits C., and hot rolled after separation from the collector. produce age hardened alloys. Variables that affect age hardening irtlclude hhcotllectoi temperature, the anneal- References Cited ing empera ure an e ime o annea ing.

The apparatus and evaporation were carried out broadly UNITED STATES PATENTS as for Example 4. In one deposition the aluminum was 1,562,202 11/ 1925 BOViIlg 117-107X was evaporated at 1550" C. in a low pressure system with 1,982,774 12/ 1934 Winkler et al. 117-107X an oxygen pressure of 10.0 105 torr. The collector was 2,353,612 7/ 1944 Gardner 75-122. at 35 C. and the original microhardness was 97 kg. 3,268,422 8/1965 E- -l- Smith et al, 204-39 mm.2. The deposit, on chemical analysis contained 12% 3,307,936 3/1967 H- R- Smith, Jr. 75-10 by weight of oxide and was annealed at 400 C. for 6 3,427,154 2/1969 Mader et al. 75-134 hours at the end of which time the hardness was 151 65 3,436,256 4/1969 Neugebauer 117-213 kg. mm.2. The change of microhardness with time on 2,967,351 1/1961 RObeltS et al. 75-138X annealing at 400 C. of this specimen is shown in FIG. 4. 3,380,820 4/1968 Hetke et al 75-138 Further examples of annealing aluminum-oxygen deposits to produce age-hardened alloys are given in Table L- DEWAYNE RUTLEDGE, Primary Examiner 2; the rst three items. When the collector is at a temperature in excess of C. the oxygen is not retained in solution but is deposited as an oxide and these deposited E. L. WEISE, Assistant Examiner U.S. Cl. X.R. 

